The field of the present invention relates to calibration of a position finding device of a radio-based positioning system, and in particular to calibration with regard to a signal strength of received radio signals.
Different positioning technologies are available for localizing mobile terminals. The probably best-known system for localization and/or navigation in the outdoor domain is the satellite-aided Global Positioning System (GPS). For localization and/or navigation within buildings, or indoors, various approaches have been known, such as infrared systems, RFID (radio frequency identification) systems, or field strength evaluations of IEEE 802.11 WLAN networks (WLAN=wireless local area network). Currently, the GPS system is reliably available only for the outdoor domain. More recent upgrades, such as highly sensitive receivers or the so-called A-GPS (Assisted GPS) represent attempts to utilize the technology also within buildings. In this context, A-GPS combines the satellite-based GPS system with reception of so-called assistance information from cellular mobile radio networks. Presently, however, these technologies do not exhibit the desired average levels of accuracy. Infrared and RFID systems are generally not available with blanket coverage and are tied to specific preconditions.
For wireless network connection of portable devices, the WLAN standard in accordance with IEEE 802.11 has been able to establish itself. It is being continuously developed further, both with regard to the data rate and to the range. Established standards, just as standards that are still in the design phase, enable broad-band data transmission at high data rates and are characterized by a high degree of integration, which enables low-cost hardware. Current PDAs (personal digital assistants) and smartphones mostly have wireless interfaces integrated therein, such as the WLAN mentioned, for example. In addition, Bluetooth is often used, and in the future possibly also WiMAX.
In the case of WLAN, commercial public WLAN access points (so-called hotspots) are now available in many places seeing many visitors. In addition, the strong increase in the propagation of broad-band internet connections (e.g. via DSL) also in private homes has boosted propagation of WLAN as a cost-effective home networking technology. Several studies have revealed that urban areas in many places today exhibit almost blanket coverage of WLAN, or even have an oversupply. In particular places of daily life and of interest to tourists are well-equipped in this respect. Therefore, it is currently convenient to use WLAN as a basic technology for positioning mobile devices. In the future, other technologies will certainly also be employed to which the inventive concept described may also be applied.
Positioning of mobile terminals in WLAN networks may in principle be performed by evaluating the base stations (hotspots or access points) received by a mobile terminal, their respective signal strengths received at the terminal being evaluated, for example. However, WLAN signals are strongly screened off by buildings and other obstacles; in particular in areas with extensive WLAN coverage, there are usually no ideal free-field conditions, since said areas are located in urban areas. Thus, it is not directly possible to infer the distance between a mobile terminal and a base station or any other communication partner from the signal strength and/or field strength measured.
In WLAN-based positioning systems, so-called received-signal-strength fingerprinting is often employed as a basic method. This radio fingerprinting method is based on the assumption that signal strengths of radio signals of several radio stations, said signal strengths being received or receivable at a current location, unambiguously characterize the current location or position. If there is a reference database which contains, for a number of reference locations or reference positions, transmitter IDs of radio stations that are received or receivable there at reference times, and contains the signal strengths of the corresponding radio signals, the current position of the mobile device may be inferred from a set of current measurement values (transmitter IDs and associated signal strength values) in that the currently measured measurement values and the reference values of the database are matched. This matching evaluates for each reference point the level of similarity of its previously recorded measurement values or reference values with the current measurement values of the current position. The reference point(s) exhibiting the highest level of similarity then form a basis of an estimated value of the current location of the mobile terminal.
For a reference database, the signal strength of a radio transmitter receivable at a reference position at a reference measuring time may be determined experimentally by means of reference measurements. This results in a database which contains, for each geographic reference position, a list of radio transmitters (access points) with the respectively associated reception field strength and quality. This list, associated with a reference position, may also be referred to as a reference measurement pack, or reference pack (RP). With a WLAN implementation, such a reference database may be as follows, for example:
RIDMACRSSIPGSXYZMAPNRCREATED100.0D.54.9E.17.8146530100579515627150012.03.07 12:42100.0D.54.9E.1A.BA6726090579515627150012.03.07 12:42100.0D.54.9E.1D.647200288579515627150012.03.07 12:42100.0E.6A.D3.B9.8B59531100579515627150012.03.07 12:42100.0F.A3.10.07.6C4646496579515627150012.03.07 12:42100.0F.A3.10.07.FB7448894579515627150012.03.07 12:42100.0F.A3.10.09.SF7237597579515627150012.03.07 12:42200.0D.54.9E.17.81541381001439915451150012.03.07 12:43200.0D.54.9E.18.1D76560111439915451150012.03.07 12:43200.0D.54.9E.1A.BA62318941439915451150012.03.07 12:43200.0D.54.9E.1D.6471348961439915451150012.03.07 12:43200.0E.6A.D3.B9.8B453931001439915451150012.03.07 12:43200.0F.A3.10.07.6C66853961439915451150012.03.07 12:43200.0F.A3.10.07.FB722511001439915451150012.03.07 12:43200.0F.A3.10.09.5F70990901439915451150012.03.07 12:43300.0D.54.9E.17.81582911002458315627150012.03.07 12:43300.0D.54.9E.18.1D78610682458315627150012.03.07 12:43300.0D.54.9E.1A.BA62153982458315627150012.03.07 12:43300.0D.54.9E.1D.6464187902458315627150012.03.07 12:43300.0E.6A.D3.B9.8B328511002458315627150012.03.07 12:43300.0F.A3.10.07.6C69006962458315627150012.03.07 12:43300.0F.A3.10.07.FB71749922458315627150012.03.07 12:43300.0F.A3.10.09.5F71482832458315627150012.03.07 12:43300.0F.A3.10.09.8071000402458315627150012.03.07 12:43
The table contains the following information:                reference position identification (ID) (here: 1, 2, 3),        MAC addresses of the radio transmitters received at the respective reference position,        the reception field strengths of the radio transmitters receivable at the reference position (RSSI=received signal strength indicator; 46.56 signifies −46.56 dBm, for example),        the reference position in Cartesian, metric coordinates (x, y, z), and        the point in time or time stamp of the measurement value capture.        
The optional PGS (percentage seen) values indicate the frequency with which the respective station was seen, expressed as a percentage, at the measurement value capture (i.e. PGS=90 means that the station was measured in 9 out of 10 measurements on average).
Additionally, a reference pack may include orientation information containing information about a spatial orientation of a mobile terminal with which the corresponding reference pack was recorded. I.e. the orientation information stores information about the direction in which a mobile terminal was held during calibration of the reference point and/or about the direction in which the mobile terminal was moved during calibration of the reference point. This is of interest particularly when the mobile terminal, such as a WLAN-capable device, is held immediately in front of the body of a user, so that the body may cause shadow effects. Even without said shadow effects caused by the body, additional orientation information may be useful since virtually every antenna has a directional pattern. Therefore, orientation may also be understood to mean, e.g., the spatial alignment of the main lobe of an antenna of a mobile terminal.
The reference data and/or reference packs described are typically created at least during a training phase during operation of the radio-based positioning system and may be acquired in different ways: e.g. manually generating the individual reference points and/or reference packs, generating reference data from a moving measurement, or generating reference data by means of anticipation data for the radio-based positioning system.
As was mentioned above, radio-based positioning by means of fingerprinting above all uses the signal strength values (RSSI) of radio transmitters for position finding. The signal strength observed by the position finding device, i.e. the signal strength as is employed for positioning purposes within the position finding device, may be subject to variations that may be due to different causes. For example, different antennas for measuring WLAN signals might possibly vary widely in terms of quality, especially if this is contemplated against the background that software that is independent of devices and platforms as far as possible is to be employed for WLAN localization on as large a variety of different types of terminals as possible. Such differences in antenna quality have an impact, among other things, on the observed signal levels of the radio signals used for positioning.
In addition to the above-mentioned differences in antenna quality, other causes may also have an impact on the signal levels of radio signals used for positioning. For example, the position finding device may occasionally be carried along in the pocket of a piece of clothing or in a piece of luggage. In such cases, the radio signals are more or less heavily attenuated by the materials of the piece of clothing or piece of luggage.
Since the signal strength and/or the signal level allows conclusions to be drawn about a distance between the position finding device and a radio transmitter transmitting the corresponding radio signal, knowledge of the actually present signal strength (i.e. without any corrupting influences due to different antenna qualities or variable attenuation) may contribute to reliably localizing a current position of the position finding device.